As Janesy stated in the third reply post. The difference between the two is that the 1187 is a gas compensating system which will allow you to fire from light 2 3/4" loads right up to the heavy 2 3/4" and 3" shells. With the 1100, if it's a non magnum receiver with a 2 3/4" chambered barrel, you can fire light loads. Heavier 2 3/4" loads will funtion without a problem, though, I personally wouldn't do that, or I would fire heavy 2 3/4" shell infrequently, because I think it causes excessive wear.
If you have a magnum receiver with a 3" barrel it should fire heavy 2 3/4" and will definatley fire 3'' loads.
Remington tried to go one step further and use the compensating system for their 3 1/2" super mags; though I haven't heard too many good things about that endeavor.